A. Field of the Invention
The present invention relates to a WSS (Wafer Support System) wafer (solid-phase bonded wafer), and particularly relates to a method for separating a support substrate from the WSS wafer and a method for manufacturing a semiconductor device using this method.
B. Description of the Related Art
As to a power device such as an IGBT (Insulated Gate Bipolar Transistor), it has been recognized that reduction in the thickness of a semiconductor substrate has a great advantage in view of characteristics such as reduction in energy loss, heat dissipation, etc. As for the reduction in the thickness of a semiconductor substrate, highest manufacturing efficiency can be obtained if a semiconductor device can be manufactured using a wafer (thin wafer) which is already thin in an initial stage where the wafer is inputted to a process. To this end, there is a strong need to establish a wafer process in which a thin wafer can be inputted without any problem. In fact, however, wafer cracking or the like increases easily due to stress caused by heat when a thin wafer is inputted in the initial stage of the process. Therefore, as shown in FIG. 3, for example, Si wafer 100a having a diameter of 8 inches and a thickness of 725 μm is generally prepared as a thick wafer, which is inputted in the initial stage of the process (FIG. 3(a)). A device structure such as an MOS (Metal Oxide Film Semiconductor) gate structure (not shown) and an emitter metal electrode (not shown) required for an IGBT is formed on one of surfaces of Si wafer 100a (FIG. 3(b)). After the device structure is formed on the front side, for example, the back side of Si wafer 100a is polished by a grinding machine and a CMP (Chemical Mechanical Polishing apparatus) in a step of thinning Si wafer 100a, which is a step close to the end of the second half of the process, to thereby make Si wafer 100a into Si wafer 100b with a thickness of 100 μm (FIG. 3(c)). Collector layer 101 and collector electrode 102 are formed on the back side of Si wafer 100b (FIG. 3(d)), which is then divided into IGBT chips 103 (FIG. 3(e)) by blade dicing. Practically such a manufacturing method using a process for finishing the wafer into a substrate with an intended thin thickness in the second half of a wafer process is often performed in order to prevent wafer cracking. As a result, 80% or more of the thickness of the inputted Si wafer is removed at the stage of a step of grinding the back surface thereof. Components derived from a grinding stone or impurity elements caused by doping are mixed in silicon dust generated by the grinding. Therefore, the silicon dust cannot be recycled.
In addition, a reinforcing rib system and a WSS system have been known as techniques for keeping the strength of a Si wafer and thinning the wafer simultaneously. The reinforcing rib system is a manufacturing method in which the inner circumferential side of the back surface of a wafer is ground and hollowed out during the aforementioned back-surface grinding while an outermost circumferential edge portion of the back surface of the wafer is left like a ring with a width of about 3 mm so that the wafer whose rigidity is kept in spite of its thin thickness can be inputted through manufacturing steps (FIG. 2). FIG. 2 shows the same steps as those in FIG. 3 except the back-surface grinding step shown in FIGS. 2(c) and 2(d).
On the other hand, the WSS system includes a temporary pasting system and a direct bonding system (solid-phase bonding system). According to the temporary pasting system for pasting a support substrate to the back surface of a wafer with an adhesive agent, high temperature processing cannot be performed after the adhesion. Therefore, in the temporary pasting system, a thick wafer is inputted at an initial stage. A support substrate is pasted to the front side of the wafer with an adhesive agent to keep the strength of the wafer in or after the second half of a wafer process where a high temperature processing step has been terminated. After that, the back surface of the wafer is ground to form a Si wafer. Necessary processing for a semiconductor device is applied to the back side of the Si wafer. After the processing, the support substrate is separated from the Si wafer.
The direct bonding system for bonding a support substrate to the back surface without an adhesive agent uses an existing Si wafer solid-phase bonding technique. When solid-phase bonding is used, high temperature processing such as an impurity thermal diffusion step in the first half of the wafer process can be performed without any problem even on the Si wafer to which the support substrate has been bonded. However, this technique is originally not based on the assumption that the support substrate will be separated again. Therefore, there is a problem in the fact that a method for easily separating the support substrate from the solid-phase bonded Si wafer has not been put into practice yet.
The strength of the Si wafer can be kept by the reinforcing rib system or the WSS system. Accordingly, reduction of a risk of wafer cracking during transportation, reduction of warp, etc. can be achieved. The WSS system or the WSS wafer that will be referred to in the following description means a system or a solid-phase bonded wafer in which a support substrate is directly bonded to the wafer by solid-phase bonding.
In addition, a technique called stealth dicing for dividing a wafer into chips has been known. The term “stealth” means hidden or invisible. According to this dividing technique, infrared laser light that can penetrate silicon is focused on the inside of the Si wafer, and the wafer is irradiated with the laser light and scanned in a grid pattern along cutting regions of chips in the wafer. Thus, the wafer surface is not damaged, but a breaking layer is formed only in the inside due to the laser light. Internal stress and cracks running in the wafer surface direction due to the internal stress are formed in the breaking layer. When external stress such as tape expansion is applied subsequently, the cracks running in the wafer surface direction can be grown so that the wafer can be divided into rectangular chips. Chipping is apt to appear in an edge surface according to existing blade dicing or the like. According to the stealth dicing technique, however, such chipping is hardly observed either in the back side or in the front side. Further, unlike surface absorption type laser machining or cutting, the stealth dicing technique has a merit that dust or the like does not scatter.